As members of the subfamily Caprinae of the family Bovidae, muskoxen are more closely related to sheep and goats than to oxen; however, they are placed in their own genus, Ovibos (Latin: "sheep-ox"). The muskox is one of the two largest extant members of Caprinae; along with the similarly sized takin.[4] While takin and muskox were once considered possibly related, the takin lacks common ovibonine features, such as the muskox's specialized horn morphology, and genetic analysis shows that their lineages actually separated early in caprine evolution. Instead, the muskox's closest living relatives appear to be the gorals of the genus Naemorhedus, nowadays common in many countries of central and east Asia. The vague similarity between takin and muskox must therefore be considered an example of convergent evolution.[5]

The modern muskox is the last member of a line of ovibovines that first evolved in temperate regions of Asia and adapted to a cold tundra environment late in its evolutionary history. Muskoxen ancestors with sheep-like high-positioned horns (horn cores being mostly over the plane of the frontal bones, rather than below them as in modern muskoxen) first left the temperate forests for the developing grasslands of Central Asia during the Pliocene, expanding into Siberia and the rest of northern Eurasia. Later migration waves of Asian ungulates that included high-horned muskoxen reached Europe and North America during the first half of the Pleistocene. The first well known muskox, the "shrub-ox" Euceratherium, crossed to North America over an early version of the Bering Land Bridge two million years ago and prospered in the American southwest and Mexico. Euceratherium was larger yet more lightly built than modern muskoxen, looking like a giant sheep with massive horns, and preferred hilly grasslands.

A genus with intermediate horns, Soergelia, inhabited Eurasia in the early Pleistocene, from Spain to Siberia, and crossed to North America during the Irvingtonian (1.8 million years to 240,000 years ago), soon after Euceratherium. Unlike Euceratherium, which survived in America down to the Pleistocene-Holoceneextinction event, Soergelia was a lowland dweller that disappeared fairly early, displaced by more advanced ungulates, such as the "giant muskox" Praeovibos (literally "before Ovibos"). The low-horned Praeovibos was present in Europe and the Mediterranean 1.5 million years ago, colonized Alaska and the Yukon one million years ago and disappeared half a million years ago. Praeovibos was a highly adaptable animal that appears associated with cold tundra (reindeer) and temperate woodland (red deer) faunas alike. During the Mindel glaciation 500,000 years ago, Praeovibos was present in the Kolyma river area in eastern Siberia in association with many Ice Agemegafauna that would later coexist with Ovibos, in the Kolyma itself and elsewhere, including wild horses, reindeer, woolly mammoth and stag-moose. It is debated, however, if Praeovibos was directly ancestral to Ovibos, or both genera descended from a common ancestor, since the two occurred together during the middle Pleistocene. Defenders of ancestry from Praeovibos have proposed that Praeovibos evolved into Ovibos in one region during a period of isolation and expanded later, replacing the remaining populations of Praeovibos.[5]

Two more Praeovibos-like genera were named in America in the 19th century, Bootherium and Symbos, which are now identified as the male and female forms of a single, sexually dimorphic species, the "woodland muskox", Bootherium bombifrons. Bootherium inhabited open woodland areas of North America during the late Pleistocene, from Alaska to Texas and maybe even Mexico, but was most common in the Southern United States, while Ovibos replaced it in the tundra-steppe to the north, immediately south of the Laurentian ice sheet.[5][6]

Modern Ovibos appeared in Germany almost one million years ago and was common in the region through the Pleistocene. By the Mindel, muskoxen had also reached the British Isles. Both Germany and Britain were just south of the Barents-Kara Ice Sheet and covered in tundra during cold periods, but Pleistocene muskoxen are also rarely recorded in more benign and wooded areas to the south like France and Green Spain, where they coexisted with temperate ungulates like red deer and aurochs. Likewise, the muskox is known to have survived in Britain during warm interglacial periods.[5]

Today's muskoxen are descended from others believed to have migrated from Siberia to North America between 200,000[7] and 90,000 years ago,[8] having previously occupied Alaska (at the time united to Siberia and isolated periodically from the rest of North America by the union of the Laurentide and Cordilleran Ice Sheets during colder periods) between 250,000 and 150,000 years ago. After migrating south during one of the warmer periods of the Illinoian glaciation, non-Alaskan American muskoxen would be isolated from the rest in the colder periods. The muskox was already present in its current stronghold of Banks Island 34,000 years ago, but the existence of other ice-free areas in the Canadian Arctic Archipelago at the time is disputed.[5]

Fossil DNA evidence suggests that muskoxen were not only more geographically widespread during the Pleistocene, but also more genetically diverse.[11] During that time, other populations of muskoxen lived across the Arctic, from the Ural Mountains to Greenland. By contrast, the current genetic makeup of the species is more homogenous. Climate fluctuation may have affected this shift in genetic diversity: research indicates colder periods in Earth's history are correlated with more diversity, and warmer periods with more homogeneity.[10]

Both male and female muskoxen have long, curved horns. Muskoxen stand 1.1 to 1.5 m (4 to 5 ft) high at the shoulder, with females measuring 135 to 200 cm (4.4 to 6.6 ft) in length, and the larger males 200 to 250 cm (6.6 to 8.2 ft). The small tail, often concealed under a layer of fur, measures only 10 cm (3.9 in) long. Adults, on average, weigh 285 kg (630 lb) and range from 180 to 410 kg (400 to 900 lb).[4][12] The thick coat and large head suggests a larger animal than the muskox truly is; the bison, to which the muskox is often compared, can weigh up to twice as much.[13] However, heavy zoo-kept specimens have weighed up to 650 kg (1,400 lb).[14] Their coat, a mix of black, gray, and brown, includes long guard hairs that almost reach the ground. Rare "white muskoxen" have been spotted in the Queen Maud Gulf Bird Sanctuary.[15] Muskoxen are occasionally domesticated for wool, meat, and milk.[16][17] The wool, qiviut, is highly prized for its softness, length, and insulation value. Prices for yarn range between $40 and $80 per ounce (28 g).[18][19][20]

A muskox can reach speeds of up to 60 km/h (37 mph).[21] Their life expectancy is 12–20 years.

After the disappearance of the Laurentide ice sheet, the muskox gradually moved north across the Canadian Arctic Archipelago, arriving in Greenland from Ellesmere Island at about 350 AD, during the late Holocene. Their arrival in northwestern Greenland probably occurred within a few hundred years of the arrival of the Dorset and Thule cultures in the present-day Qaanaaq area. Human predation around Qaanaaq may have restricted muskoxen from moving down the west coast, and instead kept them confined to the northeastern fringes of the island.[22]

In modern times, muskoxen were restricted to the Arctic areas of Canada, Greenland, and the United States. The Alaskan population was wiped out in the late 19th or early 20th century. Their depletion has been attributed to excessive hunting, but an adverse change in climate may have contributed.[16][23] However, muskoxen have since been reintroduced to Alaska. The United States Fish and Wildlife Service introduced muskox onto Nunivak Island in 1935 as a means for subsistence living.[24]

In Russia, animals from Banks and Nunivak alike were imported and released in the Taymyr Peninsula between 1974 and 1975, and from Nunivak to Wrangel Island in 1975. Both locations are north of the Arctic Circle. Once established, these populations have been, in turn, used as sources for further introductions in Siberia between 1996 and 2010.[27] One of the last of such actions was the release of six animals within the "Pleistocene Park" project area in the Kolyma River in 2010, where a team of Russian scientists lead by Sergey Zimov aims to prove that muskoxen, along with other Pleistocene megafauna that survived into the early Holocene in northern Siberia,[28] did not disappear from the region due to climate change, but because of human hunting.[29]

Ancient muskox remains have never been found in eastern Canada, despite the ecological conditions in the northern Labrador Peninsula being suitable for them. In 1967, 14 animals were captured near Eureka, Ellesmere Island, and placed in a wool farm in Kuujjuaq, northern Quebec. Although the animals acclimatized and reproduced with no problem, the farm failed to make a profit. Subsequently, 54 animals from the farm were released in three locations of northern Quebec between 1973 and 1983, while the remaining were ceded to local zoos. Between 1983 and 1986, the released animals increased from 148 to 290, at a rate of 1.25 per year, and by 2003, an estimated 1400 muskoxen were in Quebec. Additionally, 112 adults and 25 calves were counted in the nearby Diana Island in 2005, having arrived there by their own means from the continent. Vagrant adults are sometimes spotted in the continental part of Newfoundland and Labrador, though no herds have been observed in the region.[30]

During the summer, muskoxen live in wet areas, such as river valleys, moving to higher elevations in the winter to avoid deep snow. Muskoxen will eat grasses, arctic willows, woody plants, lichens, and mosses. When food is abundant, they prefer succulent and nutritious grasses in an area. Willows are the most commonly eaten plants in the winter. Muskoxen require a high threshold of fat reserves in order to conceive, which reflects their conservative breeding strategy. Winter ranges typically have shallow snow to reduce the energy costs of digging through snow to reach forage.[1] The primary predators of muskoxen are Arctic wolves, which may account for up to half of all mortality for the species. Other occasional predators, likely mainly predators of calves or infirm adults, can include grizzly bears and polar bears.[14]

Muskoxen live in herds of 12–24 in the winter and 8–20 in the summer.[31] They do not hold territories, but they do mark their trails with preorbital glands.[32] Male and female muskoxen both have separate age-based hierarchies, with mature oxen being dominant over juveniles.[31] Dominant oxen tend to get access to the best resources[14] and will displace subordinates from patches of grass during the winter.[31] Muskoxen bulls assert their dominance in many different ways. One is a "rush and butt", in which a dominant bull rushes a subordinate from the side with its horns, and will warn the subordinate so it can have a chance to get away.[33] Bulls will also roar, swing their heads, and paw the ground.[14] Dominant bulls sometimes treat subordinate bulls like cows. A dominant bull will casually kick a subordinate with its foreleg, something they do to cows during mating.[34] Dominant bulls will also mock copulate subordinates and sniff their genitals.[34] A subordinate bull can change his status by charging a dominant bull.[35]

The mating (or "rutting") season of the muskoxen begins in late June or early July. During this time, dominant bulls will fight others out of the herds and establish harems of usually six or seven cows and their offspring. Fighting bulls will first rub their preorbital glands against their legs while bellowing loudly, and then display their horns.[35] The bulls then back up 20 meters, lower their heads, and charge into each other, and will keep doing so until one bull gives up.[33] Subordinate and elderly bulls will leave the herds to form bachelor groups or become solitary.[14] However, when danger is present, the outside bulls can return to the herd for protection.[36] Dominant bulls will prevent cows from leaving their harems.[14] During mating, a bull will casually kick an estrous cow with his foreleg to calm her down and make her more receptive to his advances.[34] The herds reassemble when summer ends.[36]

While the bulls are more aggressive during the rutting season and make the decisions in the groups, the females take charge during gestation.[14] Pregnant females are aggressive and decide what distance the herd travels in a day and where they will bed for the night.[37] The herds move more frequently when cows are lactating, to allow them to get enough food to nurse their offspring.[37] Cows have an eight- to nine-month gestation period, with calving occurring from April to June. Cows do not calve every year. When winters are severe, cows will not go into estrus and thus not calve the next year. When calving, cows stay in the herd for protection. Calves are able to keep up with the herd within just a few hours after birth. The calves are welcomed into the herd and nursed for the first two months.[14] After that, a calf then begins eating vegetation and nurses only occasionally. Cows communicate with their calves through braying. The calf’s bond with its mother weakens after two years.

Muskoxen have a distinctive defensive behavior: when the herd is threatened, the bulls and cows will face outward to form a stationary ring or semicircle around the calves.[38] The bulls are usually the front line for defense against predators with the cows and juveniles gathering close to them.[14] Bulls determine the defensive formation during rutting, while the cows decide the rest of the year.[36]

The preorbital gland secretion of muskoxen has a "light, sweetish, ethereal" odor.[2] Analysis of preorbital gland secretion extract showed the presence of cholesterol (which is nonvolatile), benzaldehyde, a series of straight-chain saturated gamma-lactones ranging from C8H14O2 to C12H22O2 (with C10H18O2 being most abundant), and probably the monounsaturated gamma lactone C12H20O2.[2] The saturated gamma-lactone series has an odor similar to that of the secretion.[2]

Historically, this species declined because of overhunting, but population recovery has taken place following enforcement of hunting regulations.[1] Management in the late 1900s was mostly conservative hunting quotas to foster recovery and recolonization from the historic declines.[1] The current world population of muskoxen is estimated at between 80,000[39] and 125,000,[24] with an estimated 68,788 living on Banks Island.[40]